Air conditioners, refrigerators, and heat pumps produce a controlled heat transfer by evaporating a liquid refrigerant in a heat exchanger under appropriate pressure conditions to produce the desired evaporator temperatures. Liquid refrigerant removes its latent heat of vaporization from the medium being cooled, being converted into a vapor at the same pressure and temperature. This vapor is then conveyed into a compressor where its temperature and pressure are increased. The vapor then is conducted to a separate heat exchanger serving as a condenser where the gaseous refrigerant absorbs its heat of condensation from a heat transfer fluid in heat exchange relation therewith, changing state from a gas to a liquid. The liquid is supplied to an evaporator after flowing through an expansion device which acts to reduce the pressure of the liquid refrigerant so that the liquid refrigerant evaporates within the evaporator to absorb its heat of vaporization and complete the cycle.
During the heating mode, a heat pump circuit uses an outdoor heat exchanger coil which serves as the evaporator. The evaporator is typically located in ambient air, which sometimes drops to temperatures below the freezing point of water. Thus, as the cold ambient air circulates over the outdoor coil, water vapor in the air condenses and freezes on the surfaces of the outdoor coil. As frost accumulates on the outdoor coil, a layer of ice builds up between the portion of the outdoor coil carrying refrigerant and the air flowing over it. This layer of ice acts as an insulating layer inhibiting the heat transfer in the coil between the refrigerant and the air. In addition, the ice may block narrow air flow passageways between fins used to enhance heat transfer. This additional effect further reduces the heat transfer since lesser amounts of air are circulated in heat exchange relation with the refrigerant carrying conduits.
It is necessary to remove the accumulated frost to efficiently operate a heat pump in relatively low outdoor ambient air conditions. Many conventional methods are known such as supplying electric resistance heat, reversing the heat pump such that the evaporator becomes a condenser, or other refrigerant circuiting techniques to direct hot gaseous refrigerant directly to the frosted heat exchanger. Many of these defrost techniques use energy that is therefore not used to transfer heat energy to the space to be heated. To reduce the amount of heat energy used in the defrost operation, it is desirable to use a defrost system which places the refrigeration circuit in the defrost mode only when it is determined that too much frost has accumulated on the outdoor coil. Defrost control systems must know what a clean coil status is in order to determine the correct time to initiate a defrost cycle for best performance.
Different types of control systems are used to initiate defrost. A combination of a timer and a thermostat may be used to determine when to initiate defrost. The thermostat periodically checks to see whether or not the outdoor refrigerant temperature or a temperature dependent thereon is below a selected level, and if so, acts to place the system in defrost till the coil temperature is warmed enough to assure frost removal or for a length of time dependent on the timer. Other types of prior art defrost initiation systems include measuring infrared radiation emitted from the fins of the refrigerant carrying coil, measuring the air pressure differentials of the air flow flowing through the heat exchanger, measuring the temperature difference between the coil and the ambient air, using an electrical device placed on the fin whose characteristics change depending on the temperature of the device, optical-electrical methods and other methods involving the monitoring of various electrical parameters.
Defrost controls used today typically reset the timing function upon powering up the system. A time temperature defrost typically requires a 90 minute time cycle to elapse before allowing the system to move into defrost mode. This is usually not a problem, since the system is only powered up when initially installed or after a loss of power caused by a storm, which is very infrequent. However, electric utility companies are increasingly using power blackouts in certain areas of the country to reduce their system loading. This predominantly occurs during the colder seasons in the Southeastern United States where heat pumps and electric heating are both common. A problem results in that these power blackouts typically occur for 10 minutes each hour. Thus, the heat pump has its defrost timer reset every 60 minutes, which is less than the typical 90 minute minimum between defrost cycles. The missed defrost cycles cause the system to build up frost and suffer performance degradation, frequently resulting in service calls from the homeowner who believes that the heat pump is not working properly. **Any type of defrost control which does not include a memory of coil frost condition before shut down would require this type of early timed defrost, if the unit powers up with the coil not being proven as clear of frost.